Cold sores
Cold sores are painful blisters usually found on or near the lips. They are caused by herpes simplex virus, most often HSV-1. They can return after an earlier infection and are different from canker sores inside the mouth.
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BEYOND THE DEFINITION
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Tingling, itching, or burning may appear before small painful blisters. Blisters can break and form a crust as they heal. Illness or sunlight may trigger another episode. A sore inside the mouth or a pus-filled spot may have a completely different cause. [2]
Cold sores can spread from the first tingling until they have completely healed. Avoid kissing and oral sex during that time, and wash your hands after touching the area. Never kiss a newborn while you have a cold sore; infection can be serious. [2]
ALSO WORTH KNOWING [1]
A mouth or lip sore should not be diagnosed from appearance alone in this glossary. [1]
THE CLINICAL CONVERSATION
Ask for a clinical assessment if a sore is unusually large or painful, has not begun healing within about ten days, or keeps returning. Mention a weakened immune system and painful or swollen gums. A clinician can check the diagnosis and discuss whether treatment is appropriate. [2]
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Prepare for a care conversationTHREE QUESTIONS TO BRING
A FEW STRAIGHT ANSWERS
General information to help you prepare for a conversation about your own health.
Yes. Some treatments work best early in an episode. If sores recur, ask a clinician what to do when the first familiar warning signs appear. [2]
No. Cold sores and canker sores are different conditions. Canker sores are not contagious, while the virus behind cold sores can spread between people. [1]
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References checked November 7, 2026. This content has not been clinically reviewed for Northguard.
